In the latest post to his blog Josh Holmes talks about some of the things Microsoft is doing to contribute back to the Open Source community including code contributions and services they offer.

However, the thing that's really exciting to me is that what [Microsoft signing the Joomla! Contributor Agreement] means is that the Microsoft legal department has signed off on writing GPL'd code under the right circumstances. That's awesome! It's a clear demonstration of how far Microsoft has come in it's commitment to OSS projects.

He mentions a few of the projects they give back to including IronRuby, the Linux kernel, it's CodePlex project hosting service and many, many more (over 400 of them). There's also contributions that have been made from other teams too, like the work the SQL Server team has done on PHPBB.